docs: def _default_return_columns(self):
          """
          Return a list of the model elements that does not include lookup functions
          or other functions that take parameters.
          """
          return_columns = []
          parsed_expr = []
  
          for key, value in self.components._namespace.items():
              if hasattr(self.components, value):
                  sig = signature(getattr(self.components, value))
                  # The `*args` reference handles the py2.7 decorator.
                  if len(set(sig.parameters) - {'args'}) == 0:
                      expr = self.components._namespace[key]
                      if not expr in parsed_expr:
                          return_columns.append(key)
                          parsed_expr.append(expr)
  
          return return_columns